Techno-economic analysis on nitrogen reverse Brayton cycles for efficient coalbed methane liquefaction process

Abstract

Recovery of coalbed methane (CBM) is of great significance for environmental protection, safe production of coal mines, and clean energy supply. To reveal the efficiency difference of different nitrogen reverse Brayton cycles (N2-RBC) for recovering CBM and identify the most efficient N2-RBC in terms of energy consumption and economy, four typical N2-RBCs were studied numerically. Each process was optimized globally by performing genetic algorithm (GA) procedure, using specific power consumption (SPC) per unit product as the objective function. Pinch analysis approach was used to improve N2-RBC. All proposed processes were assessed comprehensively concerning efficiency, exergy loss, and economy. It is found that the slope of the hot/cold composite curve is greatly influenced by the material streams’ mass flow rate while slightly affected by the specific heat except for the region near critical parameters. The parallel arrangement of expanders reaches a better match between hot and cold composite curves than the series. For the liquefaction capacity of 10 tons per day, the liquefaction process using parallel N2-RBC achieves the lowest SPC of 0.72 kWh·kg−1 and the highest figure of merit of 31.11%. It also has the lowest exergy loss of 180.1 kW and a total investment of 4047.76 k$, respectively.